Although we know a great deal about vertebrate immune mechanisms, studies on the origins of such activity, which surely must reside among the invertebrates, are only in their infancy. We are proposing in depth studies on the immune capabilities of the sea star Dermasterias imbricata, since it demonstrates adaptive cell-mediated immunity, and represents an advanced invertebrate thought to be directly ancestral to the vertebrates. We propose extensive grafting studies to determine if animals at this phylogenetic level can generate long-term immunological memory. Histological and ultrastructural studies of allograft tissues, in various stages of rejection, will be carried out in order to clearly define the population(s) of effector cells mediating the response. Antigenically stressed animals will be surveyed for the possible existence of organized lymphoid-like tissue. In vitro assays will be employed to determine if sea star hemocytes can demonstrate mitogenic and mixed lymphocyte reactivity characteristics of vertebrate lymphocytes. We will also search for the existence of humoral factors analogous to those prevalent in higher vertebrates. In addition, preliminary attempts at defining the nature of cell surface receptors will be undertaken. The data from these investigations should shed a great deal of light not only on the definitions of invertebrate immunity, but also on the evolutionary transition from invertebrate to vertebrate immune reactivity.